Masked man fallacy

In philosophical logic, the masked man fallacy (also known as the intensional fallacy and the epistemic fallacy[1]) is committed when one makes an illicit use of Leibniz's law in an argument. Leibniz's law states that, if one object has a certain property, while another object does not have the same property, the two objects cannot be identical.

This is valid because being something is different from knowing (or believing, etc.) something.

Intension (with an 's') is the connotation of a word or phrase - in contrast with its extension, the things to which it applies. Intensional sentences are often intentional (with a 't'), that is they involve a property of the mind that is directed at an object.